[BUG] - <title> Missing And Incorrect Testcases

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[BUG] - Missing and Incorrect Testcases in LeetCode's "Cheapest Flights Within K Stops" Problem

As a developer, we often rely on online platforms like LeetCode to practice and improve our coding skills. However, these platforms can sometimes have flaws in their test cases, which can lead to incorrect or inefficient code being accepted as solutions. In this article, we will discuss a specific issue with the "Cheapest Flights Within K Stops" problem on LeetCode, where a solution with a logical error is being accepted due to missing test cases.

The "Cheapest Flights Within K Stops" problem is a classic graph theory problem where we need to find the cheapest price to travel from a source node to a destination node within a given number of stops (k). The problem statement is as follows:

Given n cities and m flights, where each flight is represented by an edge (u, v) with a price p, find the cheapest price to travel from city src to city dst with at most k stops.

In my implementation of the solution, I mistakenly created the adjacency list using the number of edges (flights.size() + 1) instead of the number of nodes (n). This is logically incorrect because the graph should have an adjacency list sized to the number of nodes n, not the number of flights. If any node has an index greater than flights.size(), this results in an out-of-bounds access or silently ignores connections, which is undefined behavior.

However, despite this mistake, my solution is still being accepted. This suggests that the test cases do not include flights where the node index exceeds the number of flights, which allows buggy code to pass.

Let's take a closer look at the code used for the solution:

class Solution {
public:
    int findCheapestPrice(int n, vector<vector<int>>& flights, int src, int dst, int k) 
    {
        vector<int> dist(n, INT_MAX);
        int sz = flights.size();
        vector<vector<pair<int, int>>> adj(sz + 1);   

        for(auto &it : flights)
        {
            int u = it[0];
            int v = it[1];
            int price = it[2];

            adj[u].push_back({v, price});
        }

        dist[src] = 0;
        queue<tuple<int, int, int>> q;

        q.push({0, src, 0});

        while(!q.empty())
        {
            auto [cnt, node, price] = q.front();
            q.pop();

            if(cnt > k)
                break;
            
            for(auto neighbour : adj[node])
            {
                if(dist[neighbour.first] > price + neighbour.second)
                {
                    dist[neighbour.first] =  price + neighbour.second;
                    q.push({cnt + 1, neighbour.first, dist[neighbour.first]});
                }
            }
        }

        if(dist[dst] == INT_MAX)
            return -1;
        return dist[dst];
    }
};

As we can see, the code uses an adjacency list to represent the graph, where each node is associated with a list of its neighboring nodes and their corresponding prices. The code then uses a queue to perform a breadth-first search (BFS) to find the cheapest price to travel from the source node to the destination node within the given number of stops.

The expected behavior of the code is to give an overflow error in some cases where the number of cities is greater than the flights count. However, due to the missing test cases, the code is being accepted despite the logical error.

In conclusion, the "Cheapest Flights Within K Stops" problem on LeetCode has a flaw in its test cases, which allows buggy code to pass. This highlights the importance of having strong test cases to ensure that solutions are correct and efficient. We hope that LeetCode will address this issue and provide stronger test cases to prevent similar issues in the future.

To prevent similar issues in the future, we recommend that LeetCode:

  1. Add stronger test cases to catch logical errors in solutions.
  2. Ensure that solutions using improperly sized adjacency lists are not accepted.
  3. Provide clear and concise problem statements to avoid confusion.
  4. Encourage developers to provide feedback and suggestions for improving the platform.

By following these recommendations, LeetCode can provide a more robust and reliable platform for developers to practice and improve their coding skills.
[BUG] - Missing and Incorrect Testcases in LeetCode's "Cheapest Flights Within K Stops" Problem

Q: What is the issue with the "Cheapest Flights Within K Stops" problem on LeetCode?

A: The issue is that the test cases do not include flights where the node index exceeds the number of flights, which allows buggy code to pass. This means that solutions with logical errors, such as using an improperly sized adjacency list, are being accepted.

Q: What is the expected behavior of the code?

A: The expected behavior of the code is to give an overflow error in some cases where the number of cities is greater than the flights count. However, due to the missing test cases, the code is being accepted despite the logical error.

Q: What is the impact of this issue on developers?

A: This issue can have a significant impact on developers who are using LeetCode to practice and improve their coding skills. It can lead to developers writing inefficient or incorrect code, which can be detrimental to their learning and development.

Q: What can LeetCode do to prevent similar issues in the future?

A: LeetCode can take several steps to prevent similar issues in the future, including:

  1. Adding stronger test cases to catch logical errors in solutions.
  2. Ensuring that solutions using improperly sized adjacency lists are not accepted.
  3. Providing clear and concise problem statements to avoid confusion.
  4. Encouraging developers to provide feedback and suggestions for improving the platform.

Q: How can developers help to prevent similar issues in the future?

A: Developers can help to prevent similar issues in the future by:

  1. Providing feedback and suggestions for improving the platform.
  2. Reporting any issues or bugs they encounter.
  3. Participating in discussions and forums to help improve the platform.
  4. Sharing their own experiences and knowledge with the community.

Q: What is the importance of having strong test cases?

A: Having strong test cases is crucial for ensuring that solutions are correct and efficient. Test cases help to catch logical errors and ensure that the code is working as expected. Without strong test cases, developers may write inefficient or incorrect code, which can have serious consequences.

Q: How can developers ensure that their code is correct and efficient?

A: Developers can ensure that their code is correct and efficient by:

  1. Writing clear and concise code.
  2. Using proper data structures and algorithms.
  3. Testing their code thoroughly.
  4. Seeking feedback and suggestions from others.

Q: What is the role of the community in preventing similar issues in the future?

A: The community plays a crucial role in preventing similar issues in the future. By participating in discussions and forums, sharing their own experiences and knowledge, and providing feedback and suggestions, developers can help to improve the platform and prevent similar issues from arising in the future.

In conclusion, the "Cheapest Flights Within K Stops" problem on LeetCode has a flaw in its test cases, which allows buggy code to pass. This highlights the importance of having strong test cases to ensure that solutions are correct and efficient. By following the recommendations outlined in this article, LeetCode can provide a more robust reliable platform for developers to practice and improve their coding skills.